1. Field of the Invention
The present invention relates to a method of preparing surface-imprinted polymer microspheres in the form of core-shell for the selective separation of heavy metal ions.
2. Description of the Related Art
Recently, a molecularly imprinted polymer (MIP) or a metal ion imprinted polymer (MIIP), advantageously serving as a separation tool which is environmentally friendly and simplifies the separation process, has been developed. The term “MIP” or “MIIP” refers to a polymer which includes a space having the same form as a template. The space is formed by synthesizing a polymer from a monomer that binds with a template and then removing the template.
Because only a template having the same form as the template space can be inserted therein and a molecule having a stereo-structure different from that of the template cannot be inserted therein, the use of the polymer having the template space enables the separation of other molecules that are different from the template in structure. This is the same theory as Fischer's Lock-and-Key Concept, in which an antibody specifically interacts with a corresponding antigen, or Receptor Theory, in which an enzyme has a specific activity with its counterpart substrate.
The preparation of the MIP or MIIP is described in detail below. For molecular or ion imprinting, a template is dissolved in a porogen and is then mixed with a polymerizable functional monomer having a functional group able to bind with part of the template, thus forming a composite of the template and the functional monomer. Thereafter, in order to maintain the arrangement of the functional monomer bound with the template, a cross-linker, which is an inert monomer, and a polymerization initiator are added in excess amounts, so that polymerization takes place. In this procedure, the porogen for dissolving the template plays an important role in determining the properties of the synthesized polymer. In particular, because a polar porogen dissolves a polar molecule, it may destroy the bonds between the template and functional single molecules.
Actually, with regard to MIP techniques, thorough research into structural designs and preparation methods for molecules having the ability to recognize specific molecules has been continued to date, and has mainly focused on the separation of compounds which are difficult to separate, such as racemic compounds or amino acids. The basic idea of applying the MIP techniques to the selective separation of heavy metal ions is proposed these days. At present, related research is being conducted in some advanced countries including the USA, Japan, and Sweden, and the scope thereof is further widening.
The Mosbach group of Lund University in Sweden has attempted the selective separation of various types of similarly structured compounds through molecular imprinting [Mosbach, K. Trends biochem. 19, 9 (1994)]. Further, the Murray group of Maryland University in the USA has synthesized ion imprinted polymers and thus suggested the probability of the selective separation for metal ions, such as Pb(II), Cd(II), Li(II), Na(II), Mg(II), Ca(II), Cu(II), Zn(II), and Hg(II) [Rrasado Rao, T., Sobhi Daniel, Mary Gladis, J., Trends in Analytical Chemistry, 23, 28 (2004), Yongwen Liu, Xijun Chang, Sui Wang, Yong Guo, Bingjun Din, Shuangming Meng, Analytica Chimica Acta, 519, 173 (2004)]. Furthermore, by the Fish group of Lawrence Berkely Laboratory in the USA, the selective separation of Zn(II) has been confirmed using the triazacyclononane ligand. Moreover, at Kyushu University and others in Japan, lots of efforts are directed to studying separation properties using metal ion imprinting, and some significant results have been found.
The molecular or ion imprinting techniques are a method of separating a template molecule or ion by imprinting the template molecule or ion, which is to be separated, and then removing it to thus obtain an imprinted site having the ability to recognize the template molecule or ion. The metal ion polymer is synthesized from a monomer capable of binding with a template metal ion. Depending on the type of polymer synthesis method, the form of the MIIP varies. For instance, bulk polymerization is the easiest method. However, this method is disadvantageous because it decreases the uniformity of an imprinted system, reduces the preparation yield of particles, destroys the imprinted site, requires a great amount of cross-linker, and is difficult to apply to chromatography or other industries, attributable to the non-uniform particle size or form. In addition, suspension polymerization or emulsion polymerization may overcome the aforementioned disadvantages of bulk polymerization, but requires a complicated and expensive process with the use of a specific dispersant or emulsifier, and also decreases the binding force of the imprinted polymer, attributable to an undesired interaction between the monomer and the dispersant. In contrast, MIIP, in which polymer beads having a predetermined size serve as a core and the surface of the particles is imprinted, is synthesized, thus making it possible to be efficiently applied to chromatography or other industries. Further, because there is no need for a specific dispersant or emulsifier, MIIP synthesis is advantageous from environmental and economic standpoints, and furthermore, the molecularly imprinted site is present on the surface of the beads, so that rapid diffusion between ligands is induced, resulting in good separation.